TestDerivedDetObjects: Use static methods instead of strings to access condition keys
Works around (what seems to be a) clang (mis)compilation issue with inline static strings as discussed first in
leading to in some places null strings to be used, wheres in others the string was properly set.
Workaround is to not rely on the static string, and directly call a static method instead.
@sponce FYI I don't expect the need for any ref. updates so this could go in quickly, to resolve the issues in master.